FIG. 1 is a block diagram showing a configuration of a conventional infrared camera.
Referring to FIG. 1, reference character M denotes a subject, reference numeral 1 an infrared optical system, reference numeral 2 an imaging device located on an focal plane of the infrared optical system 1, reference numeral 3 an element temperature monitor thermally connected to the imaging device 2, reference numerals 4 and 5 bias power supplies, both of which are connected to the imaging device 2, reference numeral 6 a driving circuit connected to the imaging device 2, reference numeral 7 an amplifying/display processing circuit connected to the imaging device 2, reference numeral 8 a thermoelectric device thermally connected to the imaging device 2, reference numeral 9 a constant voltage source, and reference numeral 10 a thermoelectric device driving circuit connected to the element temperature monitor 3, the constant voltage source 9 and the thermoelectric device -8. Reference numeral 11 denotes an element package for packaging therein the imaging device 2, the element temperature monitor 3 and the thermoelectric device 8, and reference numeral 12 an infrared window, which is a part of the element package 11, and through which infrared rays are transmitted. The inside of the element package 11 is kept vacuum, and an exemplary conventional technique on such a packaging method is found in Japanese Patent National Publication No. Hei 7-508384. Reference numeral 13 denotes a timing generator connected to the driving circuit 6 and the amplifying/display processing circuit 7, reference numeral 14 a body of the camera for housing therein these components 2 to 13 and reference numeral 40 a power supply circuit for supplying power required for operations of the respective components.
FIG. 2 is a block diagram showing a configuration of the imaging device 2, and 2×2 pixels are taken as an example for simplification.
Referring to FIG. 2, reference numerals 15 to 18 denote infrared detectors, reference numerals 19 to 22 diodes, reference numeral 23 to 27 transistors, reference numeral 28 a horizontal scanning circuit and reference numeral 29 a vertical scanning circuit. The infrared detectors 15 to 18 are, for example, microbolometers with hollow structure disclosed in Japanese Patent National Publication No. Hei 7-509057.
Now, the operation of the imaging device will be described. When these components are supplied with power by the power supply circuit 40, the thermoelectric device driving circuit 10 supplies the thermoelectric device 8 with power corresponding to the difference between an output of the element temperature monitor 3 and that of the constant voltage source 9 responsible for the setting of the operating temperature of the imaging device 2 to settle the temperature of the imaging device 2 at a constant room temperature. This temperature is usually in a range from 20° to 40° C.
Next, the infrared rays irradiated from the subject M are condensed by the infrared optical system 1, and then form an image on the infrared detectors 15 to 18 after passing through the infrared window 12. This slightly rises the temperatures of the infrared detectors 15 to 18 by about several mK in proportion to the intensity of the infrared rays irradiated from the subject M, and the respective resistance values of the detectors vary on an individual detector basis.
Subsequently, an element driving clock generated by the timing generator 13 is applied to the imaging device 2 from the driving circuit 6. The clock is supplied to the horizontal scanning circuit 28 and the vertical scanning circuit 29, and a current determined depending on the applied voltage of the bias power supply 5 and the characteristics of the transistor 27 is successively supplied to the infrared detectors 15 to 18 from the bias power supply 4 by successively turning on the transistors 23 to 26.
The bias current flows through one selected infrared detector and the transistor 27 into ground due to the presence of the diodes 19 to 22, and a signal corresponding to the resistance value of each infrared detector is outputted as a potential difference between the transistor 27 and the ground. The signal is inputted to the amplifying/display processing circuit 7 and outputted as a video signal.
The conventional infrared camera having been configured as above and it is obligated to operate in the temperature range about from −10° to 50° C. Thus, it takes longer time to settle the temperature of the imaging device 2 to a desired operating temperature as the ambient temperature at the power-on when the power supply circuit 40 starts supplying those components with power departs from the operating temperature set by the constant voltage source 9. This becomes longer the start-up time from the power-on to an image output.
Further, the amount of heat flowing in from or out to the outside through the thermal resistances between the thermoelectric device 8 and the imaging device 2 and between the thermoelectric device 8 and the element package 11 or the thermal resistance by electrical contact between the imaging device 2 and the outside becomes larger. This becomes larger power consumption of the thermoelectric device 8 required to settle the imaging device 2 to a constant temperature.
The present invention has been made to solve the above problems, and it is an object of the present invention to provide an infrared camera with shorter starting time and lower power consumption in a wide range of ambient temperatures by settling the imaging device 2 to a temperature obtained by adding a desired offset to the ambient temperature at power-on and by operating it.